The steam generators of pressurized-water nuclear reactors comprise a bundle of tubes bent in the form of a U, which tubes are crimped in a tube plate of great thickness at each of their ends. These tubes ensure the separation between the pressurized water forming the primary fluid and the feed water forming the secondary fluid and which is heated and vaporized in the steam generator. The pressurized primary water circulates inside the tubes, and the feed water is brought in contact with the outer surface of the tubes in part of the steam generator.
When the steam generator has been in service for a long period of time, some of the tubes of the bundle are liable to be damaged under the effect of corrosion caused by the primary fluid or the secondary fluid and show cracks in their wall. It is then necessary to put the corresponding tube out of action or repair it, in order to prevent primary fluid from being introduced into the part of the steam generator receiving the feed water.
It is known to repair the tubes of steam generators having cracked zones causing leaks by the use of a liner which is introduced into the tube in its cracked zone and which is crimped inside the tube on either side of the cracked zone.
It may be expedient to remove these liners after a certain period of service in the steam generator, for example in order to check or study the stability of the liner under the operating conditions of the steam generator.
The repair liners for the steam-generator tubes are fastened as a result of diametral expansion inside the tubes, the liner and the tube usually both experiencing plastic deformation.
The crimping of the liner in the tube is generally carried out, on the one hand, in a zone of the tube located in the tube plate and, on the other hand, in a zone located outside the tube plate. Crimping outside the tube plate causes diametral expansion of the tube, with the result that it is no longer possible to pass this crimped zone through the tube passage hole in the tube plate during dismantling for the purpose of an expert appraisal.
It is therefore possible to extract a liner only after the tube and the liner have been given a certain possibility of diametral contraction in this crimped zone outside the tube plate.
Various processes have been proposed for carrying out this contraction of the tube, for example rapid heating of the tube followed by cooling, during which the stresses exerted cause diametral contraction of the tube. However, for tubes of a diameter substantially equal to or less than 20 mm, the contraction obtainable is very low and most of the time insufficient to allow extraction under good conditions.
There has therefore been a proposal for an extraction method involving making longitudinal slits in the wall of the tube and/or of the liner in their crimped zone by cutting, and then exerting a pull on the tube and the liner which it becomes possible to pass through the holes in the tube plate as a result of diametral contraction of the zones located between the longitudinal cuts.
The cutting operation can be carried out by electrical discharge machining. For example, a technique generally known as and called electric "sawing arc" is known.
In this process, a tool comprising a rotary disc forming an electrode supplied with electrical current is introduced into the liner or the crimped tube to be removed. The rotation and radial advance of the disc make it possible to cut the wall of the liner and of the tube over its entire thickness, the tool subsequently being moved over an axial length corresponding to the length desired for the longitudinal slits. The operation is repeated a number of times, and, after a certain rotation of the tool about the axis of the tube, this is sufficient to obtain the desired number of longitudinal slits located at a distance from one another along the periphery of the tube and/or of the liner.
During the entire operation, water is circulated inside the tube and makes it possible to cool and eliminate the particles ejected by the disc during cutting.
It is clear that such a device comprising a rotating disc, means of advance, supply means and, if appropriate, guide means which have to be introduced into the tube presents difficulties in design terms, where tubes of an inside diameter less than 20 mm are concerned.
On the other hand, making several slits presupposes successive operations between which the tool must to reorientated.